(a) Field of the Invention
The present invention relates to a method of eliminating noise in measurements of very low current signals such as those with ion electrodes.
(b) Description of the Prior Art
In instruments for measuring low signals with an ion electrode or a similar means, various types of noise will degrade signal-to-noise ratio and constitute a great hindrance to enhancement of measuring accuracy. Elimination of noise is therefore an important problem for such measurements. Among various types of noise mentioned above, hum from an AC electric power source is at a known frequency and band-pass filters have conventionally been used to eliminate the hum from AC power source. However, this conventional method required a filter and, in addition, could not eliminate such noise sufficiently. The hum coming from the AC electric power source exists in a condition overlapped with DC current signal 1 as shown in FIG. 1. It is therefore conceivable to measure signal level for a short time and average the measured signal as a method to eliminate hum 2 coming from the electric power source without using a filter. However, the hum from the AC electric power source cannot be eliminated through averaging without measuring the signal level for a time which is short but far longer than the frequency of the hum. When the hum is measured for a long time, the level of the signal component may vary during the measurement, thereby making it impossible to determine it correctly at a definite moment of measurement.